A glass break detector is a common component of an intrusion detection system. Its purpose is to generate an alarm signal when glass is broken in an attempt to make entry into a room. In an acoustic glass break detector, the detection relies in whole or in part on airborne sound. Prior art acoustic glass break detectors use a single microphone in an electronic device which is mounted on a wall or ceiling within a specified distance from the protected glass. If the glass is broken the detector must generate an alarm signal. In addition, however, the detector ideally should not alarm when sounds from other sources are received. Such sounds may be produced by people or animals in the area, or by the normal operation of mechanical or electrical equipment, or by sounds generated outside the room, such as thunder or vehicles. Detectors use signal processing techniques such as spectral filtering and burst duration measurement to reject sounds from sources other than glass break. See, for example, U.S. Pat. No. 5,164,703, in which the detection algorithm requires the simultaneous presence of two bandpass frequencies. However, because many ordinary sounds are similar to the glass break signal, rejection of false alarms by means of signal processing is not perfect.
Since the glass to be protected is often located in one or two walls of the room, one method of improving false alarm immunity is to make the detector directional. It can then be installed so that the glass lies in the sensitive direction while a pan of the room containing potential false alarm sources lies in the insensitive direction. U.S. Pat. No. 4,837,558 attempts to take advantage of this principle by using a single microphone with inherent acoustic directionality. However, there are two problems with this approach. First, in normal rooms, sound undergoes reflections from the walls, ceiling, and floor. When a sound originates in the insensitive direction it will be reflected from walls in the sensitive direction. When the reflected sound arrives at the detector of U.S. Pat. No. 4,837,558 it is indistinguishable from sound that originated in the sensitive direction. Secondly, the acoustic directionality of the device in U.S. Pat. No. 4,837,558 is gradual and indefinite in limits. Some sensitivity is retained in all directions. A nearby sound from the insensitive direction may be received with the same amplitude as a distant sound from the sensitive direction. Furthermore, because the sensitivity of this device changes gradually with direction, it is difficult for an alarm installer to determine the true field of coverage of the device.
The use of microphone arrays and time-of-arrival processing to determine sound direction in fields other than acoustic glass break detection, is well known. For example, U.S. Pat. No. 3,859,621 describes an array of microphones and a processing system which produces a visual display of sound direction. Also, U.S. Pat. No. 3,715,577 describes an underwater microphone array and processing system which provides audible cues to a diver which allows the diver to determine the direction of an underwater acoustic beacon. Other prior art references in the acoustic wave detection art include, U.S. Pat. Nos. 2,470,114; 4,134,109; 4,489,442; 4,668,941; and 2,496,031; and U.S. Statutory Invention Registration H1171.